On the secant varieties of tangential varieties

Abstract

Let $X\subset \mathbb{P}^r$ be an integral and non-degenerate variety. Let $\sigma _{a,b}(X)\subseteq \mathbb{P}^r$, $(a,b)\in \mathbb{N}^2$, be the join of $a$ copies of $X$ and $b$ copies of the tangential variety of $X$. Using the classical Alexander-Hirschowitz theorem (case $b=0$) and a recent paper by H. Abo and N. Vannieuwenhoven (case $a=0$) we compute $\dim \sigma _{a,b}(X)$ in many cases when $X$ is the $d$-Veronese embedding of $\mathbb{P}^n$. This is related to certain additive decompositions of homogeneous polynomials. We give a general theorem proving that $\dim \sigma _{0,b}(X)$ is the expected one when $X=Y\times \mathbb{P}^1$ has a suitable Segre-Veronese style embedding in $\mathbb{P}^r$. As a corollary we prove that if $d_i\ge 3$, $1\le i \le n$, and $(d_1+1)(d_2+1)\ge 38$ the tangential variety of $(\mathbb{P}^1)^n$ embedded by $|\mathcal{O} _{(\mathbb{P} ^1)^n}(d_1,\dots ,d_n)|$ is not defective and a similar statement for $\mathbb{P}^n\times \mathbb{P}^1$. For an arbitrary $X$ and an ample line bundle $L$ on $X$ we prove the existence of an integer $k_0$ such that for all $t\ge k_0$ the tangential variety of $X$ with respect to $|L^{\otimes t}|$ is not defective.